1. Field of the Invention
The present invention relates to an electronic endoscope system and, in particular, to a medical electronic endoscope system which picks up images in a field sequential manner.
2. Description of the Related Art
A field sequential image pickup method, which is capable of obtaining from a single CCD two-dimension sensor color images corresponding to illumination lights of respective colors sequentially, is effective when a large number of CCD two-dimension sensors cannot be used and thus, in recent years, such field sequential image pickup method has been applied to a medical electronic endoscope system.
The electronic endoscope system of the above-mentioned type is arranged such that field sequential image signals by colors obtained by sequentially picking up images corresponding to illumination lights can be converted simultaneously by means of red, green and blue field memories and then can be reproduced on a color TV as color images. Also, in such electronic endoscope system, if a freeze instruction to make the monitor images stand still is applied, then the memory images of the red, green and blue field memories are latched and the still images can be monitored. Further, if such electronic endoscope system is connected to a hard copy device and a recording instruction is given, then the still images can be hard copied.
In field sequential image pickup method, the color images in the respective color channels are produced each delayed by a frame, and they are converted simultaneously and are then reproduced as the color images. For this reason, in a conventional electronic endoscope system, if the monitor images are caused to stand still by the freeze instruction, then color discrepancies may occur in the still monitor images because of movements between an object to be photographed and the leading portion of the endoscope.
Then, conventionally, in order to prevent the occurrence of the color discrepancies in the still monitor images, there have been proposed several electronic endoscope systems (see Japanese Patent Application Laid-open (Tokkai) No. 2-41131, (Tokkai) No. 2-41132 and (Tokkai) No.2-55030) in which, if a freeze instruction is input, then it is checked whether the image being currently picked up is a still image or a moving image, and only when it is decided to be the still image, the storage images of the red, green and blue field memories are latched so as to obtain a still image having no color discrepancies.
However, in the above-mentioned conventional electronic endoscope systems, if the image being currently picked up is decided to be a moving image when the freeze instruction is input, the stored images of the respective color field memories may not be latched and thus the still image cannot be obtained immediately. In this case, a time discrepancy occurs from the output of the freeze instruction to the time when a still image with no color discrepancy can be obtained actually.